There has been known a technique in which an NOx sensor, a reducing agent supply device, and an NOx selective reduction catalyst are arranged in an exhaust passage of an internal combustion engine in a sequential manner from an upstream side thereof to a downstream side thereof (for example, see a first patent document).
Here, even if the NOx sensor is arranged at the upstream side of the reducing agent supply device, there is a fear that urea water as a reducing agent supplied from the reducing agent supply device may flow backwards to the vicinity of the NOx sensor due to the pulsation of exhaust gas, etc. When ammonia (NH3) derived from urea adheres to the NOx sensor or exists in the vicinity of the NOx sensor, the ammonia may be detected by the NOx sensor, similar to NOx. On the other hand, the ammonia adhered to the NOx sensor may reduce NOx, thereby decreasing the concentration of NOx. If doing so, it will become difficult to detect the concentration of NOx in the exhaust gas in an accurate manner.
In addition, in cases where a temperature sensor is provided, if a liquid reducing agent is adhered to the temperature sensor or exists in the vicinity of the temperature sensor, heat in the surroundings of the temperature sensor is taken up by the reducing agent upon evaporation thereof, so it becomes difficult to detect the temperature of the exhaust gas in an accurate manner.
Further, in cases where a catalyst using HC as a reducing agent is provided, when the HC is added from the reducing agent supply device, it becomes difficult due to detect the air fuel ratio of the exhaust gas in an accurate manner due to the HC thus added.
The additive added in the exhaust gas adheres to a sensor in this manner, or there is a fear in the vicinity of a sensor that an error may occur in the detected value of the said sensor when it exists.